1. Field of the Invention
This invention relates to low-pressure electric discharge lamps and has particular reference to an improved fluorescent lamp in which the mercury-vapor pressure is regulated by means of an amalgam.
2. Description of the Prior Art
Low-pressure mercury-vapor discharge lamps that contain a strategically located metal (such as indium, cadmium, etc.) which forms an amalgam with mercury and regulates the mercury-vapor pressure within the lamp during operation are well known in the art. A fluorescent lamp of this type having an amalgam-forming metal located on the inner surface of the phosphor-coated envelope is described in U.S. Pat. No. 3,007,071 issued Oct. 31, 1961 to A. Lompe et al.
It is also known in the art that the time required for a fluorescent lamp to reach full light output from a "cold start" can be reduced by placing an auxiliary quantity of amalgam at a location within the lamp where it will be quickly heated and thus rapidly supply mercury-vapor to the discharge. A lamp having such an auxiliary amalgam disposed on one of the anodes near the cathode is disclosed in U.S. Pat. No. 3,227,907 issued Jan. 4, 1966 to C. J. Bernier et al.
A high-output fluorescent lamp having a continuous coating of amalgamative metal on the tubular portions of the glass stems is disclosed in U.S. Pat. No. 3,287,587 issued Nov. 22, 1966 to R. A. Menelly.
A fluorescent lamp containing an amalgamative metal that is divided into two flat strips which are held in place on the stem by a wire-mesh collar assembly is disclosed in U.S. Pat. No. 3,534,121 issued Oct. 13, 1970 to G. S. Evans. An improved lamp of this type in which the amalgamative-metal strips are retained in place on the stem within the wire-mesh collar assembly by an overlying strip of fine-wire mesh is disclosed in U.S. Pat. No. 3,422,299 issued Jan. 14, 1969 to C. Morehead, the author of the present invention.
While the aforesaid prior art innovations achieved the desired primary objective of controlling the mercury-vapor pressure within the lamp when the latter is operated under high ambient-temperature conditions or at high power loadings, they are not entirely satisfactory from a cost or manufacturing standpoint since they require relatively expensive components and time-consuming manual operations.
Applying the amalgamative metal to the stem in the form of a continuous coating poses another problem in that it limits the quantity of metal that can be placed at the proper location within the lamp. Larger stems or thick coatings must therefore be used in lamps of long length that require large quantities of amalgam. Thick coatings are impractical since they permit the fluid amalgam to drip from the stem and thus become dislocated.
There is, accordingly, a need for a lamp of the amalgam type that has a minimum number of parts and can be efficiently assembled on a mass-production basis. It would also be very desirable to have an amalgam vapor-regulation means that could be employed on the short stems customarily used for standard 40 watt T12 fluorescent lamps and which would also permit large quantities of amalgamative metal to be applied directly onto the stems at the proper location.